Typical HVAC systems include an air delivery system for delivering the conditioned (cooled, dehumidified, or heated) air to the room or other space in the structure. Many air delivery systems include at least one centrifugal fan unit, and may include a plurality or “array” of such fan units. Most systems are designed to reduce air flow to the space being air conditioned when the space reaches the target temperature, that is, when the load requirements of the space are reduced. Conventionally, there are two main types of systems for reducing air flow through a centrifugal fan.
Some years ago, disk unloaders were commonly used to control the amount of air passing through the centrifugal fan unit. In this type system, a disk inside the inlet funnel of the fan housing is moved by a mechanical actuator that is controlled by a system pressure sensing device. In most systems of this type, the disk unloader completely controlled the system airflow because it was movable between a completely open position, in which air flow through the fan was unobstructed, and a closed position, in which the disk was seated in the inlet funnel completely sealing off air flow. The disk unloader offered good efficiency and sound characteristics.
In most current systems, a variable speed control or drive (“VSD”) is used to control air flow through the fan unit by controlling the fan motor speed. VSD's are intrinsically more efficient than the disk type unloading devices and have essentially eliminated the disk unloading device as a device for controlling air volume.
Currently, disk unloader devices are used mainly as shut-off devices for fan systems. For example, in systems comprising a fan array, it is often necessary to turn off one or more of the fans for air flow control as well as for maintenance or repair of the fans. When a fan in a multiple fan system is powered off, air flows backward through that fan, reducing the effectiveness of the remaining operating fans. To improve the system efficiency, backflow must be prevented in the powered-off fan(s), and the disk unloader serves this function well. Backflow is eliminated by driving the disk to the closed or sealed position.
Although VSD's are the most efficient way to control airflow in response to changes in system load requirements, they have one significant disadvantage. Reduction in air flow volume is directly proportional to the fan speed reduction. However, reduction in air flow pressure is proportional to the fan speed reduction squared. This would work well if the system pressure requirements varied with flow in the same way. Most variable airflow systems have terminal devices that increase their flow resistance as the airflow requirements go down. The pressure at lower flows is always higher than it would be than it would if it varied only with the square of the flow reduction.
Additionally, there exists a large area of the potential fan operating range in which the fan exhibits unstable operating characteristics. This area is called the surge region; if the fan is caused to operate in this region, it will operate roughly with increased noise levels and may experience premature bearing failure. The fan can not be allowed to operate in the surge region.
FIG. 1 is a graph illustrating the operational parameters of a typical fan operating curve. As the flow is reduced, the system pressure follows the “System” curve. As seen in FIG. 1, the system curve crosses the minimum flow curve. This is the point where the fan starts to operate in the surge region. At lower speeds the fan will be in surge. This point, therefore, represents the minimum operating flow of the fan. In the system depicted in FIG. 1, the full flow of the fan is 8,500 cfm. The minimum speed is 5,200 cfm or sixty-one percent (61%) of full flow or thirty-nine percent (39%) turndown. If the system needs to operate at lower flows, this fan selection is unacceptable. As is also shown in FIG. 1, the fan pressure at forty percent (40%) speed and lower is lower than the system pressure requirements.
To achieve higher fan turn down a smaller fan would be selected. This would lower the system curve which would increase the fan turn-down. Unfortunately, selecting a smaller fan would result in a reduction in fan efficiency, but this is a trade-off commonly made to increase the operating range of a fan system. In order to increase the fan turndown to sixty percent (60%), the fan selected would often be two fan selections lower than the optimum. This often resulted in a loss of fan efficiency, which required the use of larger motors, which in turn increased the electrical demands of the system on the building.
In summary, though fan unloading with VSD's is more efficient, it has a limited unloading range and requires less than optimum fan selections. The disk unloading system, on the other hand, does not have a limited unloading range. Because the pressure is created by the fan speed, which does not vary in a disk unloading system, the air pressure is unaffected by the reduced air flow. This relationship is illustrated in FIG. 2, which shows the performance of system using a disk unloading device.
As seen in FIG. 2, when using a disk unloading system, the fan can be selected closer to the surge line, which allows fan selection to be optimized. As the air flow through the fan is reduced (due to reduction in available wheel width), the surge region is reduced as well. For this reason, the fan can not go into surge as the disk unloads the fan wheel.
This system has the advantage, as compared to the VSD system, that it can unload over a very wide range with much higher pressures at low flow. On the other hand, it has the disadvantage that it requires much higher fan power as it unloads, as compared to the VSD system. Moreover, conventional disk unloader devices include expensive linear actuators that are difficult to interface with many building control systems.
There is a need for a fan unit with improved efficiency and a wide unloading range. There is a need for a disk unloader with a simple and unobtrusive actuator mechanism. There is a need for a disk unloader with an actuator that requires no additional power. These and other needs are met by the present invention as will be apparent from the following description of a preferred embodiment.